Tracing the Path between Mushrooms and Alzheimer’s Disease—A Literature Review

Alzheimer’s disease (AD) is well-known among neurodegenerative diseases for the decline of cognitive functions, making overall daily tasks difficult or impossible. The disease prevails as the most common form of dementia and remains without a well-defined etiology. Being considered a disease of multifactorial origin, current targeted treatments have only managed to reduce or control symptoms, and to date, only two drugs are close to being able to halt its progression. For decades, natural compounds produced by living organisms have been at the forefront of research for new therapies. Mushrooms, which are well-known for their nutritional and medicinal properties, have also been studied for their potential use in the treatment of AD. Natural products derived from mushrooms have shown to be beneficial in several AD-related mechanisms, including the inhibition of acetylcholinesterase (AChE) and β-secretase (BACE 1); the prevention of amyloid beta (Aβ) aggregation and neurotoxicity; and the prevention of Tau expression and aggregation, as well as antioxidant and anti-inflammatory potential. Several studies in the literature relate mushrooms to neurodegenerative diseases. However, to the best of our knowledge, there is no publication that summarizes only AD data. In this context, this review aims to link the therapeutic potential of mushrooms to AD by compiling the anti-AD potential of different mushroom extracts or isolated compounds, targeting known AD-related mechanisms.


Introduction
Neurodegeneration comprises a series of illnesses that cause the gradual impairment of cognitive and motor abilities. Alzheimer's disease (AD) is a neurodegenerative disease and the most common form of dementia (60-80% of worldwide dementia cases) [1]. Statistics show that in 2018 more than 8 million people over the age of 30 suffered from AD in Europe (the number is expected to double by 2050), and in the USA, data from 2023 point to nearly 6.5 million persons over 65 years to be affected with AD (incidence of 1 in more than the treatment of AD act as cholinesterase inhibitors, NMDAR antagonists, and anti-Aβ antibodies. Donepezil, galantamine, and rivastigmine are cholinesterase inhibitors that were found to improve cognition [9,17,18], especially when used as adjunctive with the NMDAR antagonist memantine [19]. Aβ antibodies are gaining more relevance in the treatment of AD, with lecanemab, aducanumab, and donanemab being recently approved by the Food and Drug Administration (FDA). These antibodies, when used in prodromal AD cases, reduce cognitive decline caused by senile plaque formation and improve memory over the span of six months [20,21]. Despite the beneficial effects of the described anti-AD pharmaceuticals, they tend to cause harsh adverse effects, ranging from general gastrointestinal imbalance, abdominal pain, stomach ulcers, and gastritis to confusion, insomnia, hallucinations, arrhythmia, and convulsions [22][23][24][25]. To reduce inflammatory responses by the brain cells, nonsteroid anti-inflammatory drugs, such as ibuprofen, can be used. These can reduce Aβ formation with minor side effects, such as stomach aches and kidney problems [26]. Unfortunately, due to a lack of human testing and general information, there are currently no sufficient data to support nonsteroid anti-inflammatory drugs use in AD patients [27].
Natural products have been used as traditional medicines for centuries. The scientific community is taking advantage of ancient knowledge by investigating the potential protective effect of natural compounds against AD pathogenesis. For instance, the neolignan 4 -O-methylhonokiol, a constituent of the bark of Magnolia officinalis Rehder & E. H. Wilson, was found to inhibit astrocyte activation in preselin-2 mutant (PS2) mice brains and other signaling cascades involved in inflammation and oxidative stress, diminishing memory impairment [28]. Caffeic acid, a phenolic compound naturally present in plants, was shown to have neuromodulatory activity via the regulation of LPS-induced neuroinflammation through tumor necrosis factor-α reduction [29]. The protective effect of caffeic acid against AD was assessed in vivo in a high-fat hyperinsulinemic-induced rat model. The administration of 30 mg/kg of caffeic acid for 30 weeks was shown to ameliorate the memory of the animals, as well as affected the major biochemical targets of AD: decreased the expression of phosphorylated (p)-tau protein, attenuated APP expression, and regulated Aβ deposition [30]. Moreover, the compound 2,2 ,4 -trihydroxychalcone isolated from Glycyrrhiza glabra L. was administrated to a double transgenic mouse model used to study early onset AD. The model APP/PS1 mutant mice express a chimeric mouse/human amyloid precursor protein and a mutant variant of human presenilin 1, both connected to the central nervous system. The compound administered at a 9 mg/kg/day (total treatment 106 days) dosage decreased Aβ production and senile plaque formation, which resulted in memory improvement [31]. Natural compounds with antioxidant properties were shown to prevent free radical formation, thus having a potential neuroprotective effect. The most common antioxidants used in AD treatment are resveratrol, apocynin, and other flavonoids which prevent the inactivation of acetylcholine receptors [32][33][34].
A major challenge regarding AD is, in fact, the search for efficient therapies. Presently, most researchers and companies are searching for strategies to discover new targets and new drugs for therapy. The application of natural compounds is among the new features for therapies against AD, as described in recent reviews [25,35,36]. In this field, the potential role of mushrooms for AD treatment or prevention, has been highlighted [37]. Studies using mushroom extracts and compounds have demonstrated neuroprotective, neuromodulatory, and immunomodulatory activities, as well as an anti-inflammatory and antioxidant potential in neurodegenerative diseases [37][38][39]. Well know mushroom species such as Ganoderma lucidum (Curtis) P. Karst., Cordyceps militaris (L.) Fr. and Cyathus africanus H.J. Brodie have been extensively studied with reference to compounds such as ganoderic acid A (3), cordycepin (13), and cyathins (5, 6), respectively ( Figure 1). Although there are several studies in the literature focused on the potential of mushroom extracts and compounds in AD, to the best of our knowledge, there is no publication that specifically compiles information on this subject. Considering the pertinence of the research that links mushrooms to AD, we aim with this review to gather knowledge on the application of mushrooms in AD by compiling the existing data to date. pertinence of the research that links mushrooms to AD, we aim with this review to gather knowledge on the application of mushrooms in AD by compiling the existing data to date.

Mushrooms in Alzheimer's Disease
The number of articles linking mushrooms to AD prevention and/or treatment has increased, especially in the last decade. In fact, it was found that mushroom extracts and compounds were efficient against the main anti-AD-related mechanisms, including the inhibition of BACE1 [10,12], the inhibition of the Aβ aggregation, and the inhibition of AChE and butyrylcholinesterase (BChE) (Figure 1). These can be combined with complementary therapeutic approaches, such as anti-inflammatory or antioxidant therapies, which are known to play an important role in neurodegeneration [13,14,26,40]. Mushroom compounds with anti-AD potential are classified into several classes, including lanostanes, terpenoids, indole alkaloids, purine nucleosides, amino acids, lactones, benzaldehyde derivates, dienamides, and ethanolamines as presented in Figures 2 and 3.

Mushrooms in Alzheimer's Disease
The number of articles linking mushrooms to AD prevention and/or treatment has increased, especially in the last decade. In fact, it was found that mushroom extracts and compounds were efficient against the main anti-AD-related mechanisms, including the inhibition of BACE1 [10,12], the inhibition of the Aβ aggregation, and the inhibition of AChE and butyrylcholinesterase (BChE) (Figure 1). These can be combined with complementary therapeutic approaches, such as anti-inflammatory or antioxidant therapies, which are known to play an important role in neurodegeneration [13,14,26,40]. Mushroom compounds with anti-AD potential are classified into several classes, including lanostanes, terpenoids, indole alkaloids, purine nucleosides, amino acids, lactones, benzaldehyde derivates, dienamides, and ethanolamines as presented in Figures 2 and 3.

Inhibition of BACE1 and Prevention of Aβ Aggregation and Aβ Cytotoxicity
It has been demonstrated that several mushroom extracts and isolated compounds have anti-BACE1 and anti-Aβ potential (Table 1). Aβ peptide is formed through a cleavage process of APP by the secretases beta (BACE1) and gamma. Aβ peptides tend to interact with each other forming deposits that lead to the formation of amyloid plaques (also referred to as senile plaques), which are considered a hallmark of AD. An excess of Aβ peptide is secreted and accumulates near the synapse. Aβ-dependent neuronal death is thought to be induced due to changes in membrane fluidity and integrity, as these oligomers change the phosphatase and kinase activity, possibly contributing to the hyperphosphorylation of Tau protein. BACE1 has been studied and is considered a promising therapeutic target, as it is involved in Aβ formation [41]. Molecules 2023, 28, x FOR PEER REVIEW 5 of 31 Figure 2. Examples of compounds isolated from mushrooms with potential for AD treatment. These bioactive molecules belong to different classes: lanostane triterpenoids (1-3), polyoxigenated cyathane diterpenoids (4-9), sesterterpenoids (10), indole alkaloids (11 and 12), and purine nucleosides (13).

Inhibition of BACE1 and Prevention of Aβ Aggregation and Aβ Cytotoxicity
It has been demonstrated that several mushroom extracts and isolated compounds have anti-BACE1 and anti-Aβ potential ( Table 1). Aβ peptide is formed through a cleavage process of APP by the secretases beta (BACE1) and gamma. Aβ peptides tend to interact with each other forming deposits that lead to the formation of amyloid plaques (also referred to as senile plaques), which are considered a hallmark of AD. An excess of Aβ peptide is secreted and accumulates near the synapse. Aβ-dependent neuronal death is Considering the inhibition of BACE1 and the prevention of Aβ aggregation and Aβ cytotoxicity, species such as Cordyceps militaris, Ganoderma lucidum, Hericium erinaceus (Bull.) Pers. and Pleurotus ostreatus (Jacq.) P. Kumm. are the most representative ones.
The mushroom C. militaris has been considered one of the oldest sources of bioactive compounds [42]. The first report on the isolation and characterization of C. militaris metabolites dates to the 1950s and is related to the purine nucleoside cordycepin (13) (Figure 2) [43]. Since then, this molecule has been tested for multiple different purposes, including neurodegenerative diseases, as it exhibits neuroprotective, anti-inflammatory, and immunomodulatory activities [44].
The potential of cordycepin (13) for AD has been evaluated in vivo using an Aβ 1-42induced mouse model where mice were orally administered 100 and 200 mg/kg/day of cordycepin (13) for 15 days [45]. The animals treated with this compound displayed improved cognition, spatial memory, and general behavior (similar results were observed for both concentrations). Rat hippocampal neurons were used as an in vitro model to test cordycepin (13) anti-Aβ potential. The results showed that this compound can inhibit Aβ 25-35 production, as well as Aβ-induced ROS and cytotoxicity, p-tau expression, and AChE activity [46].
The medicinal fungus G. lucidum has been used in many countries for health promotion [47]. Wang et al. [48] conducted a study to assess the potential effects of diet supplementation with 0.3, 0.6, and 1.8% of G. lucidum in senescence-accelerated mice prone 8 (SAMP8). Their results showed no significant differences in body weight, eating habits, or locomotion between the treated and control groups (casein diet). However, a significant difference was observed in the behavioral tests, since the supplemented group showed quicker responses in avoidance tests. A significantly higher SOD, GSH-Px, and GSH-Rd activity, as well as the lower accumulation of Aβ was found in the test groups, showing that G. lucidum could be an interesting source of anti-Aβ compounds. Moreover, triterpenoids isolated from G. lucidum were discovered to reduce neuronal apoptosis by inhibiting the ROCK-signaling pathway using APP/PS1 mice [49].
Commonly known as Lion's Mane Mushroom, H. erinaceus is frequently used in traditional medicine, due to several health benefits such as anti-inflammatory, antibacterial or anticancer properties [50]. H. erinaceus extracts are commercialized as dietary supplements. Several bioactive molecules have been characterized being the most relevant for neurodegeneration erinacine A (7) and S (10), hericenones (19)(20)(21) (Figure 3), and dilinoleoyl phosphatidylethanolamine (22). The protective effects of erinacines A (7), C (9), and S (10) were tested in in vivo using the double transgenic mouse model APP/PS1. The animals were treated with ethanolic mycelium extracts of H. erinaceus containing these terpenes at 300 mg/kg/day for 30 days (pathological experimental group) and 70-90 days (behavioral experimental group) [51]. The pathological experimental group showed that the treatment schedule led to an inhibition of Aβ deposition, smaller plaque formation, improvement in the Aβ-degrading enzyme neprilysin, enhancement in nerve growth factor (NGF) production, and a promotion in hippocampal neurogenesis, when compared to control. The mice from the behavioral experimental group showed an improvement in daily life tasks, exhibiting higher nesting scores and nestlet shredding. A similar study conducted in SAMP8 fed with erinacine A (7)-enriched mycelium, showed a decrease in Aβ aggregation with an improvement in cognition, memory, and delay in cases of age-related cognitive decline [52]. Tzeng [53] tested the in vivo efficiency of erinacines A (7) and S (10) in APP/PS1 mice. The results showed that both compounds, at concentrations of 10 and 30 mg/kg/day (100 days) reduced Aβ plaque formation, improved plaque degradation, reduced glial activation, and promoted neurogenesis in the highest concentration. Erinacines A (7) displayed the most promising results, with a decrease in Aβ accumulation by inhibiting Aβ production. Behavioral tests also showed an improvement in burrowing and nesting behavior, as well as an increase in spatial learning and memory. The species P. ostreatus is the second most popular and most cultivated edible mushroom worldwide. Besides being appreciated for its organoleptic properties, P. ostreatus has been found to induce different bioactivities, such as antimicrobial, antitumor, antioxidant, and immunomodulatory activities [66]. Zhang et al. [64] tested the potential of P. ostreatus polysaccharides to prevent and treat AD. The results of this study showed that administration of 400 mg/kg of polysaccharides extract inhibited BACE1 and Aβ deposition in over 50%, as well as causing a significant inhibition of APP, AChE, and p-tau protein in AlCl 3 -and D-galactose-induced AD Wistar rats, in which aluminum works as a neurotoxic and D-galactose as a senescence stimulating agent, inducing AD-like symptoms. These improvements were studied through behavior tests and revealed that rats treated with P. ostreatus polysaccharides had a significant improvement in memory and navigation. Another relevant compound for AD produced by P. ostreatus is ergothioneine (14), this sulfur-containing amino acid is known for its antioxidant and cytoprotectant properties.
Ergothioneine (14) was tested in the 5XFAD AD transgenic mouse model, characterized for overexpressing five of the known familial AD mutations related to presenilins 1 and 2 that lead to an increased Aβ deposition, higher plasma, and brain levels of Aβ 40 and Aβ 42 , along with elevated pro-inflammatory cytokine levels [67]. Results showed ergothioneine (14) to have metal chelating activity by capturing heavy or potentially toxic metals in the blood stream and removing them from circulation, as well as ROS scavenging activities and the mitigation of Aβ aggregation, which may explain the improvement of the cognitive deficits observed in the experimental group compared to untreated controls [65].

AChE and BChE Inhibition
One of the most common approaches in AD treatment has been based on repairing cholinergic dysfunction. Cholinergic neurons are involved in several cognitive functions such as memory, attention, learning, and sleep regulation. AD patients have dysfunction in these neurons, mostly in the hippocampus and the neocortex. Damaged cholinergic neurons lead to a decrease in the production of acetylcholine (ACh), a neurotransmitter involved in synapses and general cognition. By lowering the activity of enzymes involved in the degradation of ACh, such as AChE and BChE, the levels of available ACh in the synaptic cleft increase [9,68,69]. When submitted to treatment with ChEIs such as galantamine, a natural compound isolated from Galanthus nivalis L., mice showed an improvement in day-to-day tasks, such as burrowing activities, as well as an enhancement in spatial memory and problem-solving activities [70,71].
Several studies with mushroom extracts and compounds were directed to the anti-AD potential by the evaluation of the AChE and BChE inhibition and the activation of ChAT. In Table 2 the main studies found in the literature are presented.
Ganoderma lucidum aqueous extracts have been shown to be neuroprotective by displaying interesting antioxidant and anti-inflammatory activities in vivo and in vitro, as well as inhibiting AChE activity [72]. Considering compounds, eighteen different lanostane triterpenes were isolated from G. lucidum fruiting bodies. The compounds were tested in vitro and showed AChE inhibition (IC50 ranging between 9.40 and 31.03 µM) [73]. The same study showed that two of the tested compounds, lucidenic acid N (1) and lucidadiol (2) also inhibited BChE, presenting IC50 below 200 µM [73]. This study showed that G. lucidium triterpenes could be potential candidates for anti-AD drugs.   Amanita caesaria (Scop.) Pers., commonly known as Caesar's Mushroom, is a wellappreciated edible mushroom commonly consumed in Turkey and is known to have significant antioxidant and antimicrobial activities, as well as a considerable fatty acid composition [86]. A. caesaria aqueous extracts were tested using the immortalized mouse hippocampal cell line HT22 and AlCl 3 , D-gal induced Balb/c mice [55]. Biochemical examinations found that the extracts promoted cell survival and decreased the expression levels of phosphorylated protein kinase B (p-Akt) and the mammalian target of rapamycin (p-mTOR), known to be associated with cellular death, most efficiently at 50 and 100 µg/mL. The in vivo assays consisted of the intragastrical administration of 250, 500, and 1000 mg/kg/day of extract for 28 days. Vertical movement, locomotor activities, and spatial memory were improved when compared to the AD control group (intragastrically administered saline solution). The inhibition of AChE activity was also observed, as well as an increase in ACh and ChAT. The treatment group was also found to have lower levels of Aβ in the brain and reduced levels of ROS and SOD.
The genera Tricholoma comes from a diverse mushroom family that is composed of edible and non-edible specimens. Several Tricholoma species were found to have antibacterial activity against Gram-negative bacteria [87,88]. In a study developed by Tel et al. [85], the antioxidant and AChE inhibition properties of methanolic, n-hexane, and ethyl-acetate extracts from T. fracticum (Britzelm.) Kreisel, T. terreum (Schaeff.) P. Kumm. and T. imbricatum (Fr.) P. Kumm. were evaluated. Results showed that T. imbricatum hexane extract was the most promising, displaying an inhibitory activity of 71.8% for AChE and 52.6% for BChE at 0.2 mg/mL.

Tau Protein Expression and Aggregation
Tau is a protein involved in microtubule formation and function. This protein plays a significant role in cytoplasmatic transport and allows synaptic function as well as the maintenance of their structure, regulating neuronal signaling. The hyperphosphorylation and abnormal cleavage of Tau leads to deficient microtubule activity due to depolymerization consequently causing a loss of neuronal morphology as well as impairments in axon and dendrite formation [6,11]. NFTs are then formed, causing a neuronal malfunction due to impaired synaptic function and neurotoxicity. The level of Tau hyperphosphorylation seems to correlate with the severity of the AD scenario in a direct way [6,11]. In Table 3, the most relevant studies linking mushrooms to protein Tau are compiled followed by a description of the most representative species.
Armillaria mellea (Vahl) P. Kumm. is an edible medicinal mushroom used in traditional medicine in East Asia [90]. Polysaccharides isolated from A. mellea have been reported as having interesting bioactivities, such as antioxidant activity due to radical scavenging potential. A study conducted by An et al. [57] intended to better understand the mechanisms behind A. mellea polysaccharides protection by using an in vitro and an in vivo approach. In vitro, results showed that these molecules improved cell viability in mouse hippocampal neuronal cell line HT22 by inhibiting caspase-3 activity, restoring mitochondrial membrane permeability, and reducing ROS accumulation. As for the in vivo approach, D-galactose-induced Balb/c mice were treated with 25 and 100 mg/kg/day for four weeks. The supplementation with A. mellea polysaccharides regulated AChE, ACh, and ChAT and Aβ concentration in mice serum and hypothalamus (Tables 1 and 2) as well as a significant reduction in p-tau aggregations in the hippocampus of treated mice at 25 and 100 mg/kg/day. Pleurotus Ostreatus (Jacq.) P. Kumm.
As already described, cordycepin (13) isolated from C. militaris was found to inhibit Aβ-induced apoptosis in hippocampal cultivated neurons and to inhibit AChE activity. At 10 µM, the compound also decreased p-tau expression through an adenosine A1 receptordependent mechanism.

Other Activities for General Neuronal Protection
Neuroprotection is generally defined as a physiological or induced measure to preserve neurons. This can be achieved through a therapeutic approach resulting in the salvage, recovery, or even regeneration of the central nervous system cells, structure, and function. Classical examples of neuroprotectors are molecules that intervene in neurotransmitter receptors pathways via agonist/antagonist interaction. These have been explored for their neuroprotective activity as well as their disease-modifying potential. Table 4 summarizes the most relevant mushroom extracts and isolated compounds with neuroprotective potential other than the effects described in the classical AD hypothesis and antioxidative and anti-inflammatory potential.
Grifola frondosa (Dicks.) Gray is an edible mushroom found in America, Europe, and Asia with nutritional and medicinal properties due to its rich composition in bioactive molecules, namely polysaccharides, β-glucans, and heteroglycans [91]. Similarly to members of the Ganoderma genus, G. frondosa is known for its medicinal properties and is used to treat diverse ailments by indigenous Malaysian tribes [92]. An in vivo study by Ling-Sing Seow et al. [93] explored the neuroprotective activities of G. frondosa, G. lucidum, and G. neo-japonicum Imazeki aqueous extracts by testing different concentrations until 2500 ganoderma neo µg/mL in the rat pheochromocytoma cell line PC12, known to have similar behaviors as mature neuronal cells [94]. The results revealed that all extracts stimulated NGF production. While G. frondosa and G. lucidum extracts stimulated NGF production at 12.60 and 12.07% at 75 µg/mL, G. neo-japonicum stimulated NGF production at 14.0% at 50 µg/mL. Lysophosphatidylethanolamine (17), a phospholipid isolated from G. frondosa, was shown to not only stimulate NGF but also enhance neuronal outgrowth and neurofilament M expression while stimulating cell proliferation and survival through the activation of the Ras/MAPK signaling pathway [95].
The lanostane triterpenoid ganodenic acid A (3) isolated from G. lucidum has been shown to inhibit cell apoptosis in brain and liver tissues through the mTOR pathway, while also promoting autophagy, when administered in concentrations of 10 mg/kg/day, when compared to control (treated with normal commercial feed) [96].
As previously described, inflammatory and oxidative processes have been revealed to be highly deleterious in neurodegeneration scenarios. It is known that mushrooms are composed of many compounds with antioxidant and anti-inflammatory molecules, such as polysaccharides, β-glucans, and phenolic/aromatic compounds. A sizable number of mushroom-isolated compounds display anti-inflammatory and antioxidant activity; therefore, most extracts and compounds described in the previous tables display such activities, thus enhancing the potential of mushroom-derived compounds as possible anti-AD treatment or adjuvants [115]. A recent review by Abitbol et al. [37] efficiently covers the relationship between neuro-inflammation and the potential use of mushroom-derived compounds in AD, for this reason, the topic was not discussed in the present review.
A multi-target therapeutic approach is believed to be more effective due to the multifactorial nature of AD and the fact that some of the hallmark mechanisms are closely related to the point of complementing the pathways involved in other mechanisms [116]. As presented in Tables 1-4, several mushroom extracts and isolated compounds display one or more anti-AD activities. There are a few examples of extracts displaying more than one activity, including the A. camphorata mycelium and fruiting body 95% ethanolic extract, which reduced Aβ-induced cytotoxicity, oxidative stress, and p-tau expression [56]. G. lucidum fruiting body aqueous extract, was shown to inhibit Aβ-induced cytotoxicity, AChE, and caspase 3-like activity, thereby enhancing cell viability as well as NGF expression [61]. Tremella fuciformis Berk. fruiting body aqueous extract inhibited cholinesterase activity while improving neuritogenesis by stimulating NGF and FGF expression [84]. The most notorious anti-AD mushroom-derived compounds are cordycepin (13) isolated from C. militaris, neocyathins B (5) and J (6) from C. africanus, and erinacine A (7) from H. erinaceus. Cordycepin (13) caused an inhibition in Aβ 25-35 production, while reducing Aβ-induced ROS, toxicity, AChE, and tau, resulting in an improvement in spatial memory when administered to mice [46]. Compounds 5 and 6 significantly inhibited Aβ 1-42 production, as well as Aβ-induced iNOS and AChE activity [60]. Erinacine A (7) inhibited Aβ deposition, Aβinduced iNOS, and microglial activation while stimulating neurogenesis through enhanced NGF production [53]. Isolated polysaccharides have also shown interesting combined bioactivities, such as P. ostreatus polysaccharides that displayed neuroprotector activity, inhibiting Aβ formation, APP expression, BACE1 activity, and p-tau deposition [64]. The most interesting results have been found in extracts compared to isolated compounds and compound groups, which can possibly be explained by existing synergistic interactions between compounds in the more diverse extract components.
The results mentioned above highlight the known anti-AD potential exhibited by some extracts and isolated compounds from mushrooms. Although the presented results appear to be interesting, there is still a lack of knowledge about the mechanisms underlying anti-AD protection, which enhances the need for additional research and new isolated molecules that can be applied to anti-AD therapeutics.

Medicinal Mushrooms in AD Clinical Studies
Mushroom extracts and derivates have been widely accepted as supplements and are thought to improve general health in different ways, such as diminishing chemotherapy side effects; helping with insomnia, anemia, or neurasthenia; and even improving memory [117,118].
Despite the evidence of the anti-AD potential of mushroom extracts and isolated compounds obtained in pre-clinical trials, there is still a lack of published clinical trials.
Some exceptions are clinical trials where patients were treated with Hericium erinaceus. In a double-blind, parallel-group, placebo-control trial that included 30 patients suffering from mild cognitive impairment, 1000 mg of 96% pure mushroom powder was administered three times a day for 16 weeks [119]. Follow-up was performed for 4 weeks after the last dosage and cognitive function was evaluated using a cognitive function scale based on the revised Hasegawa dementia scale. Results showed that, compared to the placebo group, the experimental group exhibited an improvement in cognitive function.
Although, at the end of the follow-up, there was a significant decline in results, no major adverse effects were reported with only one participant leaving the study with stomach pain complaints. Additionally, in a more recent study conducted by Li et al. [120] aiming to assess H. erinaceus mycelium enriched with erinacine A (7) potential against early AD, a randomized, double-blind, placebo-controlled pilot study was conducted for 49 weeks. In this trial, patients were administered three capsules daily containing 350 mg of H. erinaceus mycelium and 5 mg/g of compound 7. When compared to the placebo control, the results pointed to an improvement in the instrumental activities of the daily living score as well as Mini-Mental State Examination score, and contrast sensitivity. Concerning AD-specific markers, the experimental group was found to have reduced apolipoprotein APOE4 expression as well as lower concentrations of Aβ 1-40 and higher concentrations of brain-derived neurotrophic factor. Four patients left the trial after suffering from abdominal discomfort, nausea, and skin rash. Although not directly connected to AD, during a study concerning the effects of compound 7 enriched H. erinaceus on hearing impairment, Chan et al. [121] found that the administration of 2000 mg/day combined with 10 mg of compound 7 caused a significant increase in serum NGF and brain-derived neurotrophic factor. All of these studies indicate that this medicinal mushroom could be useful as a possible treatment or adjuvant for anti-AD treatment.

Conclusions
Given the incidence, severity, and complex nature of Alzheimer's disease, all efforts made towards finding a cure are critical. Mushrooms are a source of pharmacologically diverse and interesting molecules, produced by different species and body parts. By linking these two subjects, the present review provided an overview of the potential anti-AD properties of mushrooms. Throughout the literature, the link between mushrooms and the disease was obvious and the number of compounds and number of extracts with activity falling within the AD main hypotheses were surprising.
AD is a complex disease with multiple factors involved, which makes it challenging to determine the most crucial mechanisms. However, multi-target inhibitors appear to be the most promising option, as observed in some of the mushroom-derived compounds. These compounds have the potential to be used in anti-AD therapy or as adjuvants in existing treatments. However, although mushroom extracts and derived molecules have been extensively investigated both in vivo and in vitro, currently few clinical trials have been conducted.
Global mushroom production has increased in recent decades and is expected to further increase in the future. As mushrooms are widely consumed all over the world, and their consumption is well accepted when introduced into the market, the knowledge that they can play a positive role in the process of neurodegeneration and cognitive health is valuable and reinforces the necessity of research in this area. Funding: This work is funded by the European Regional Development Fund (ERDF) through the Regional Operational Program North 2020, within the scope of Project GreenHealth-digital strategies in biological assets to improve well-being and promote green health, Norte-01-0145-FEDER-000042.
Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.

Conflicts of Interest:
The authors declare no conflict of interest.